The present invention relates to the technical field of offshore rig construction and more particularly to the integration of the topside of a rig to its lower supporting hull structure for drilling or production operations. The topsides concerned substantially depend on the drilling and production requirements.
Two major parts of an offshore rig are: 1) a topside, which houses devices, equipment and crew accommodation unit needed for drilling or production of oil and gas; and 2) a lower hull which provides the necessary buoyancy to support the rig at an offshore site. The topside and hull of the rig are usually fabricated separately for many reasons (e.g. cost, schedule, capability and availability of fabrication facilities). The topside and the hull are transported to a mating site (usually required to be well protected from heavy traffic and weather environment) and integrated together through attachment of the topside onto the top of the hull. Traditionally, the topside integration with the hull is done either at the offshore site using lift vessel(s) or in smaller modules at a quay site.
Integration offshore is restricted by limited good weather window in which the environmental condition is mild enough to allow a safe operation and the availability of heavy lift vessels to lift the topside and place it on top of the hull. The offshore integration also requires a large logistic support including transportation barges, offshore tugs, supply vessels and anchor handling tugs (AHTs). Offshore integration is technically very challenging and the cost exposure is also very high. Integration in smaller modules at quay site is safer and less dependent on the weather condition, but requires sub-integrations (connections and hook ups among the modules) and commissioning works at height. The availability of quay site facilities is also very limited.
Five common basic types of topside integration methodologies have been used:
Offshore float-over. This operation involves submerging lower hull using ballast water at a pre-selected offshore location with sufficient water depth. When in position and ready to receive the topside, a heavy-lift barge carrying the topside is towed and maneuvered into position for the lower hull to be de-ballasted and mated with the topside. This is an intricate operation and highly dependent on the weather condition. It requires a large logistic support including heavy-lift transportation barges, offshore tugs, supply vessels, AHTs and a team of very experienced crew with specially trained skills. The cost of an offshore float-over operation is very high.
Use of offshore heavy lift vessel. This operation involves the use of an offshore heavy lift vessel which is basically a crane on a floating vessel (barge). The lower hull is usually brought to installation location either by dry or wet tow. The lower hull is first positioned at its final position. The topside is then lifted by the offshore heavy lift vessel and placed onto the lower hull. Depending on the total weight of the topside and the lift vessel's capacity, the integration of the topside to the hull may be done in one of the following manners:
If the lift capacity of a single lift vessel exceeds the weight of the whole topside, the whole topside is lifted by the single lift vessel and placed onto the lower hull.
If the weight of the whole topside exceeds the capacity of a single lift vessel, the topside is built and brought to the installation site in modules and each module is lifted using the single lift vessel. This operation requires connecting and hooking up the modules on site. The process takes a longer time than one single lift. Heavy lift vessels are usually hired at a fixed day rate; therefore, longer installation time means high cost.
If the weight of the whole topside exceeds the capacity of a single lift vessel, the whole topside is lifted by two or more lift vessels at the same time and placed onto the lower hull. The operation is very intricate and requires very large logistic support. The cost of this operation is also very high.
Other disadvantages of using offshore vessel lift vessels relate to the requirement that the lifting points be built into the topside structure. Also, due to limited heavy lift vessels in the world, vessels need to be pre-booked in advance, which makes the scheduling of the installation even more difficult in addition to the weather condition. Offshore lifting may be further limited by crane outreach and vessel stability, resulting in the lift vessels' maximum lifting capacity not being fully used and more lift vessels may be needed for the installation.
Integration using heave lift device at quay site. During the integration, heavy lift devices are usually huge cranes that stand on ground while the lower hull floats in water by the lift device. Depending on the weight and size of the topside, the heavy lift device can either make it in one lift or the topside has to be brought in several modules. The single lift is largely limited by the crane's outreach capacity and weight.
Integration on land with use of strand jacks. This method of integration requires a large open space with strong load bearing ground. The location requires a launching capacity. The construction of the topside and lower hull is done in the same location in pre-determined positions. Generally, the lower hull is assembled around the topside to minimize skidding distance. When all components are complete, the topside is raised off the ground with the use of strand jacks and the lower hull skidded underneath of the topside deck. Once in position, the deck is lowered to complete the integration. This method of integration requires a good load bearing ground and large land space since both the topside and the hull have to be constructed in the same location.
The use of specialty vessels. At the moment, there are several specialty vessels in construction in various parts of the world. They all have varying operation philosophy as compared to the heavy lift vessels. These specialty vessels all require certain level of offshore logistic support and are weather dependent while carrying out the installation offshore. Some of these vessels may have restriction on the footprint size of the deck, lift height and weight.
One common disadvantage of the above integration methodologies is that the weight of the mass structure is supported by a small/limited number of lift points. Each lift bears a very large load, which would result in high stress on the structure in an area around the lift point and strong structural reinforcement is needed to avoid damage during the lifting operation. If one of the lift points fails during the operation, the load it bears would transfer to other lift points. Because of the small number of lift points, the percentage of the load increase on the remaining lift points would be very significant and may cause another lift point to fail. The load on the remaining lift points would further increase, resulting in a chain reaction: all the lift points would fail one after another and the structure being lift would eventually fall causing a serious accident. To avoid the chain reaction, the lift points must be reinforced with very large safety margin. The cables/ropes used to lift the structure must also be chosen with a large safety margin. This means an inefficient use of the structural materials and high costs.
Integration with single lift using one single lift vessel/device in a protected area certainly has many advantages over multi-lift integration, especially because it requires a shorter time and simpler operation, and has lower probability of failure. As oil and gas exploration and production goes to deeper and deeper water and desired production rate increases, the weight and size of the topside of a rig (new-build or conversion) for deep water will significantly increase. The topside of such rig may weigh up to 24,000 tons or higher. Today, the largest offshore lift vessel in the world has a lift capacity of 14,000 tons (Meerema Thialf). The capacity of the largest existing heavy lift device for quay site integration available from Kiewit Offshore Services located at Ingleside, Tex. is 13,000 tons. At present, no single lift integration is possible to lift topside weighing more than 14,000 tons with the existing heavy lift vessel or device. New heavy lift device/systems with much larger lifting capacity are needed for single lift integrations. Building the new lift vessels/devices by simply scaling up the existing ones would not be economical because the size and weight of the new build would dramatically increase and require a larger operation space and a dramatically larger logistic support. Besides, building such a giant lift device itself is a challenge and very costly. Such larger lift vessels/devices would also be much more difficult to mobilize.